The present invention relates to a connecting rod of the kind in rock drilling operations, both with regard to underground and surface drilling. The invention also relates to a method of producing such a rod.
When drilling in rock, connecting rods, or extension pieces, are used between drill bit and shank adapter to enable holes of desired depths to be drilled. The rods are connected to one another and to the drill bit and shank adapter by means of screw joints.
The rods are produced in mutually different lengths with regard to underground drilling, normally lengths of up to about 3 m, and up to about 6 m with regard to surface drilling. A typical rod will have an external thread at one end and an internal thread at its other end. The rod is straight between the two threaded ends.
In the case of surface drilling, the connecting rods used are normally in the form of hot-rolled hollow rods which have been surface hardened at their thread-carrying ends. However, it has been found problematic to achieve satisfactory surface hardening of the internal thread, owing to the difficulty in obtaining a uniform profile depth. In the case of underground drilling, it has therefor been preferred to have a case hardened internal thread, which makes it necessary to case harden the entire connecting rod and therewith increase production costs.